1. Field of the Invention
This invention relates generally to feedback control systems for electronic ballasts and more particularly to a sequential feedback system which provides soft start and dimming control for an electronic ballast.
2. Description of the Related Art
Feedback control systems are used to control electronic ballasts because they provide effective control despite irregular characteristics of lamp loads, and because they are effective for reducing energy consumption and extending lamp life.
A sequential feedback control system that can implement soft start and dimming operations provides further benefits in terms of energy efficiency and extended lamp life. During a soft start operation, a lamp is driven at a reduced power level during a preheat cycle before the entering a discharge mode. This reduces the stress on the filament and extends the lifespan of the lamp. The power to the lamp is then increased until it enters a discharge mode. After a sufficient period during which the discharge is maintained, the power can be reduced during a dimming operation to adjust the brightness of the lamp commensurate with the ambient lighting conditions, thereby conserving power. A sequential control system employing feedback can adapt the power flow to the changing load conditions presented by a lamp as it is operated in the various modes.
A prior art feedback control system for an electronic ballast is shown in FIG. 1. An electronic ballast 12 drives a lamp 11 and generates a current consumption signal (ifb) which is indicative of the current consumed by the ballast. The current consumption signal (ifb) is multiplied with a direct link voltage signal (E) by multiplier 13 which generates a control current signal (imo) representative of the power consumption of the ballast 12. The direct link voltage signal (E) is converted into a direct link current signal (Ie) by a resistor (1/RL).
A resistance block (Rmo) converts the current signal into a voltage signal (Vmo) which is subtracted from a reference voltage signal (Vref) from a reference voltage generator 14 by an adder 15. The adder 15 generates an error signal (Verr) which is converted to an amplified current signal (Iin) by an error amplifier 16 having a transconductance (Gm). The amplified current signal (Iin) charges the capacitor (C) to generate an integrated voltage signal (Vin) which is changed into a integrated current signal (i1) by a voltage controlled current source (VCCS).
A second adder 18 subtracts the integrated current signal (i1) from the sum of the direct link current signal (Ie) and a standard current reference signal (Iref), thereby generating a composite current signal (it). The composite current signal (it) is used by an oscillator & output driver 19 to charge a capacitor (Ct) and generate a frequency signal (f1) which controls the power consumption of the electronic ballast (12).
Thus, the power input to the electronic ballast is controlled in a closed loop manner because the input power which is controlled by the frequency signal (f1) is proportioned to the current consumption signal (ifb).
Although the feedback control system of FIG. 1 is effective for maintaining the discharge of a lamp after it is initiated, it is difficult to adapt it to a sequential feedback control system that implements soft start and dimming functions.
Accordingly, a need remains for a feedback control system for an electronic ballast which overcomes the problems discussed above.